1. Field of the Invention
The present invention relates to a contrast medium injection device used for X-ray CT image diagnosis, MRI image diagnosis and the like.
2. Description of the Related Art
A contrast medium is used for the diagnosis of X-ray CT (computed tomography) images, MRI, angio-images (angiographic images) and the like. The contrast medium is a liquid having high viscosity and the injection thereof by means of a manual power takes a lot of time and labor so that in recent years automatic contrast medium injection devices have come to be used.
An automatic injection device 100 of FIG. 8 is an example of such devices, and since it can be mounted with two syringes, it is referred to as a double-head type. FIG. 9 shows a typical mechanism of the automatic injection device. A syringe 1a for the contrast medium is set at the side of a head A, a syringe 1b for a physiological saline solution is set at the side of a head B, and a Y-shaped tube 2 is connected to the tips of the two syringes. A catheter is connected to the tip of the Y-shaped tube and can be injected with the contrast medium and the physiological saline solution.
The physiological saline solution is used mainly for flushing inside the tube in order to prevent blood from coagulating inside the catheter and the tube after the contrast medium was injected. It is also used for the purpose of diluting the contrast medium.
As for the essential action of the device, injection of a required amount of the contrast medium is performed by forwarding a syringe piston of the head A while a syringe piston at the head B stopped, and then after the side of the head A is stopped, the side of the head B is moved forward to perform flushing with the physiological saline solution. In order to dilute the contrast medium, both cylinder pistons of the head A and the head B are moved forward so as to mix the liquid of both heads A and B in the Y-shaped tube (i.e. three way-branched tube).
In the automatic injection device of FIGS. 8 and 9, rotations of motors 4a, 4b at the sides of the head A and the head B, respectively, are transferred to motor gears 6a, 6b via gear heads 5a, 5b, and transferred to screw gears 7a, 7b linked to ball screws 8a, 8b by being reduced to a predetermined gear ratio to rotate the ball screws 8a, 8b. Furthermore, the rotation is converted into a linear movement by ball nut units 9a, 9b which are engaged with the ball screws 8a, 8b so that piston holders 3a, 3b which hold the syringe pistons are allowed to move forward or backward.
However, since the contrast medium has high viscosity, and high pressure is necessary for the injection, specifically when the contrast medium is injected, the high pressure is also transferred to the side of the head B via the Y-shaped tube. Therefore, in the case of the device using a mechanism having an extremely small frictional factor such as that of the ball screw, there was the possibility that the syringe piston at the side of the head B is pushed and forced to move backward by high pressure and the contrast medium is sucked by the head B.
For this reason, an idea can be conceived that a valve is provided between the Y branch of the Y-shaped tube and the head B so that the valve is closed when the syringe piston at the head B is in a stopped state. However, when a manually operable valve is used for this purpose, the switching operation of the valves is complicated and the switching is sometimes forgotten. Although it is possible to perform the switching of the valves automatically and electrically, it is not preferable to provide a drive unit of such a switching in the midway through a substantially soft and light tube because the balance of the device configuration become worse.
On the other hand, if a one-way valve is used, the device can be made simple and compact, but the backward-moving action of the syringe piston cannot be performed. Although an idea can be also conceived that a pressure in the forward direction is applied to the syringe piston at the head B so as to resist the pressure from the side of the head A when the syringe piston at the head B is in a stopped state, the axis of rotation of the motor continues to be in a stopped state while electricity is being supplied to the motor, and there arises a problem of the seizing of the motor.